The present invention generally relates to the field of video transmitters, and more particularly to a wideband precision phase modulator for utilization in video transmitters.
A desire existed for a wideband precision phase modulator for utilization in video transmitters. Typical implementations of wideband precision phase modulators comprise analog voltage controlled phase shifters, I/Q modulators, or wideband phase locked loops. All of the prior art analog modulator designs are adversely affected by environmental conditions such as temperature and vibration, especially when wide modulating frequency bandwidths are required. However, a completely digital implementation of a wideband precision phase modulator would not suffer from adverse vibration and temperature environmental conditions.
The principle of modulating a direct digital synthesizer (DDS) for phase or frequency modulation is well known. However, where high precision wideband modulation is desired (i.e. greater than 10 megapixels per second of video) the existing DDS designs do not meet stringent design criteria.
The specifications for high precision wideband video dictate extremely low spurious levels. For an arbitrary tuning frequency, the spurious levels of available DDS designs do not meet these requirements. The DDS is further required to have a high clock frequency to properly sample the input modulation, equivalent to meeting a Nyquist criteria, and to provide an output high enough in frequency to avoid multiple frequency upconversions to the final frequency band. Available DDS devices either provide an adequate number of phase modulation bits but have an insufficient overall clock frequency, or provide an adequate clock frequency but have an insufficient number of modulation bits.